This invention relates to microwave finline devices for signal detection and the like and more particularly to millimeter wave finline structures using integrated capacitor technology. The invention is particularly useful for detection for microwave energy having a fundamental frequency of above about twenty-five GHz.
Heretofore, most microwave waveguide detection devices have employed precision machined conventional waveguide technology. The accuracy of machining of parts becomes of critical importance with shorter wavelengths of interest. For example, wavelengths of interest include those on the order of five (5) mm. at about sixty (60) GHz. A significant problem with detectors for such high frequencies and short wavelengths is inherently poor impedance match between detection diodes and the waveguide, which results in loss of power as represented by a VSWR as great as 3:1. Other problems will be apparent hereinafter.
Because of further problems with respect to the structure of conventional waveguide detectors involving high precision probes and cavity shaping, it has been suggested that finline technology be employed. One such suggestion is found in a paper published by Holger Meinel and Lorenz-Peter Schmidt of AEG-Telefunken entitled "High Sensitivity Millimeter Wave Detectors using Fin-Line Technology", Conference Digest of Fifth International Conference on Infrared & Millimeter Waves, Wuerzburg, West Germany, 1980, pages 133-135. Therein the authors suggest the use of a millimeter wave detector using finline technology in which a Schottky diode is used as a detection element. The structure uses a quartz substrate mounted in a waveguide.
FIG. 1 herein represents a finline structure 10 reconstructed from the brief description in the prior art Meinel et al. paper. It shows a dielectrically loaded finline circuit 12 on a quartz dielectric substrate 14 in a waveguide 16. (Interior waveguide boundaries are shown partially in phantom. In the cited publication, surface and waveguide boundaries are not illustrated.) Metallization layers 18, 19 on the front surface 21 of the dielectric substrate 14 are shown to be provided, the layers 18, 19 having in surface pattern an input taper 20 and an output taper 22. Metallization layer 18 is presumed to be in d.c. contact with the waveguide 16, and metallization layer 19 is presumed to be d.c. isolated from the waveguide 16. Detected signals are presumably obtained from metallization 19. At the point of minimum exposed dielectric width 23 there is shown a junction between first metallization layer 18 and second metallization layer 19 through a zero-bias Schottky diode 24. An absorber 26 is provided according to the Meinel et al. description on the back surface of the substrate 14 which is applied along a straight taper. It is assumed the absorber 26 provides for progressive absorptive termination of the waveguide. No provision appears to have been made therein for impedance matching of the substrate 14 directly with the enclosing waveguide. Moreover, there is no suggestion for enhancements to the detection circuit, other than the use of a diode.
Heretofore it has not been possible to selectively bias multiple circuit elements of finline structures because of the difficultly in providing lossless r.f. continuity while at the same time maintaining d.c. isolation between traces in the finline structure. In the past, bias has been applied to a finline structure by biasing the entire fin by an external d.c. supply. Wave traps in the form of polyiron cavities have been provided in the waveguide forming structures to inhibit undesired reflections. Because an entire fin is biased at the same potential, all circuit elements across a finline gap are necessarily biased equally. Thus the known technique is primarily limited to use with two-terminal devices.
Matching the impedance of a free-space waveguide to a finline structure is important. Various techniques have been proposed. For example, quarter-wave transition matching transformers have been proposed. Such a technique is discussed in Verver et al., "Quarter-Wave Matching of Waveguide-to-Finline Transitions," IEEE Transactional on Microwave Theory and Techniques, Vol. MTT-32, No. 12, December 1984, pp. 1645-1647. Therein it is suggested that the transition from free space to dielectric loading of the waveguide cannot be reflectionless because of the discontinuity introduced by the dielectric. The proposed solution, namely a quarter-wave matching stub extending along the waveguide axis into the free-space waveguide from the finline structure, provides an inherently narrow frequency match. There is thus a need for a solution which offers broadband impedance matching.
While finline technology appears to provide promise, characteristics heretofore assumed to exist for dielectric materials have suggested against certain types of structures. Accordingly, the present invention is directed to advancing the state of finline technology to increase versatility and usefulness over the art heretofore known.